The present invention relates to a corona discharging apparatus in which a linear electrode is stretched across a case and, more specifically, to a corona discharging apparatus used in an image forming apparatus such as an electrophotographic copying machine, facsimile, printer, etc., and adapted to charge or discharge an image bearer such as a photosensitive body.
In electrophotographic copying machines, corona discharging apparatuses are used for charging and discharging a photosensitive body on which an image is to be formed.
In the corona discharging apparatuses of this type, an opening is formed in one face of an aluminum case, and a linear discharge electrode faces the opening. The linear discharge electrode is generally formed of corona generating wire such as tungsten, molybdenum, platinum or other precious-metal wire.
The tungsten and molybdenum wires have advantages in being low in material cost and having high tensile strength and ductility. Thus, they provide a discharge electrode which can be stretched with high tensile force for higher discharge stability without unevenness in discharge.
As regards corona generation stability, the tungsten and molybdenum wires exhibit quite satisfactory corona generating capability in the initial stage. After a very short period of operation, however, the output or the distribution and amount of corona produced become uneven and unsteady, resulting in early electrical breakdown. Therefore, the tungsten and molybdenum wires have only a very short working life.
The cause of the aforesaid drawback lies in that the surface of the corona generating wire is susceptible to oxidation. Due to oxidation, corona is radiated unevenly and will deteriorate rapidly. Consequently, it is impossible to obtain a stable and uniform corona discharge, which results in indistinct images.
If the tungsten or molybdenum wire is used in a copying machine with a silicon heat roller, its surface will be soiled and corroded by silicon vapor or other products in the machine.
If the surface of the corona generating wire is thus soiled, it is cleaned by using cotton or the like to restore it to its original condition. However, the tungsten and molybdenum wires cannot enjoy a satisfactory cleaning effect for improved life performance.
Unlike tungsten and molybdenum wires, platinum wire, which has high chemical stability, exhibits uniform and stable discharging capability. Even if its surface is soiled by any products in the copying machine, the platinum wire can easily be cleaned by means of cotton or the like. Periodic cleaning will ensure the platen wire of a very long working life.
However, the platinum wire is expensive. Also, it is much lower in tensile strength than tungsten or molybdenum wire and is highly ductile, so that it cannot be stretched with great tensile force. Namely, an increase of tensile force will result in deformation of the wire material. For example, if a high AC voltage is applied to the discharge electrode, the electrode will vibrate, degrading discharge stability.